1. Field of the Invention
The present invention relates to an image forming apparatus such as wet type electrophotographic printer, which uses a high density liquid developer. In particular, the invention is directed to an image transfer member for use in transferring a developer image between a photoconductor and a final image receiving medium such as a record paper, and to an image transfer device. The invention is also directed to an image forming system employing the image transfer member and the image transfer device.
2. Description of the Related Art
In general, an image forming apparatus such as a color electrophotographic printer forms an electrostatic latent image on each of a plurality of photoconductors such as photoconductive drums, develops the electrostatic latent image on each photoconductor using a developer of a predetermine color, and transfers the developed images to an image receiving medium such as a paper. Such a color electrophotographic printer is classified into a dry type or a wet type depending on the nature of developer used in the printer. A wet type printer uses a liquid developer containing powdered toner mixed with a volatile liquid carrier as developer.
A wet type electrophotographic printer using a liquid developer uses a toner having a particle size in the range of about 0.5 to 5 μm, which not only makes it possible for the printer to produce an image that is superior in image quality compared to an image produced by a dry type printer using powdered toner at the time of developing an electrostatic latent image, but also makes it possible to prevent a user from suffering harm caused by harmful toner dust. For these reasons, the use of wet type electrophotographic printers are gradually increasing.
In order to print out a color image, such a wet type color electrophotographic printer requires an image forming process for each of a plurality of colors, e.g., yellow, cyan, magenta and black. Therefore, in order to increase print-out speed, it is essential to reduce a length of time required for the image forming process for each color.
In order to reduce a length of time for the image forming process, there has been proposed a wet type color electrophotographic printer for forming a color image by one pass process using four photoconductors respectively forming developer images, of which colors are different from each other, and an intermediate or image transfer medium (ITM) such as a conductive belt or drum for transferring the developer images each formed on one of the four photoconductors to a image receiving medium.
In such a printer, the four photoconductors form a first transfer nip while being in contact with the ITM. If the ITM is a belt type, a plurality of first transfer rollers for applying a first bias voltage to the ITM are positioned on the inner surface of the ITM opposite to the outer surface of the ITM which is in contact with each of the four photoconductors, wherein the first bias voltage renders the developer images each formed on one of the four photoconductors to be transferred to the ITM. In addition, a second transfer roller is positioned at one side of the outer surface of the ITM, wherein an image receiving medium is sandwiched between the outer surface and the second transfer roller. The ITM and the second transfer roller form a second nip between them. The second transfer roller applies a second bias voltage to the image receiving medium, wherein the second bias voltage renders the developer images formed on the ITM to be transferred to the image receiving medium.
Therefore, when the ITM rotates, the developer images respectively formed on the individual photoconductors are transferred onto the ITM in an overlapping manner by the first bias voltage of the first transfer rollers in the first transfer nip section, and the developer images overlapped on the ITM are transferred onto the image receiving medium by the second bias voltage of the second transfer roller.
In a similar manner, the wet type color electrophotographic printer forming a color image in a one pass process may employ a conductive belt or drum as the ITM. However, the belt (hereinbelow, referred to as “ITB (Image Transfer Belt)” is more commonly employed than the drum because the former requires a small installation space as compared to the latter and allows the printer be more freely designed.
However, such a wet type color electrophotographic printer employing an ITB requires a high electric force at the time of transferring a developer image formed on a photoconductor to the ITB or an image receiving medium because the developer image contains high density toner particles having a particle size in the range of about 0.5 to 5 μm and electrified with a high electric charge in the range of about 2 to 10 times of the electric charge of dry type developer (50 μC/g), that is, with a high electric charge in the range of 100 to 500 μC/g.
For example, in order to transfer a developer image formed by a liquid developer having a high electric charge in the range of 100 to 500 μC/g from a photoconductor to an ITB in a narrow first transfer nip, there is typically needed a first bias voltage of not less than about ±1.2 KV. Therefore, the electric potential prior to transferring the developer image from the ITB to an image receiving medium in a second nip will be equal to or higher than about ±400 V. As a result, in order to transfer the developer image from the ITB to the image receiving medium, a second bias voltage of not less than about ±5 KV is needed, whereby the developer image transferred from the ITB to the image receiving medium will contain transfer defects such as transfer voids due to an air gap or breakdown produced depending on the density of the developer image under a high voltage of not less than about ±5 KV and the transfer efficiency will be deteriorated.
Therefore, it is desired to maintain the second bias voltage at a low level not exceeding about ±5 KV in order to prevent the above-mentioned problems. However, if the first bias voltage and/or second bias voltage are lowered not to exceed ±1.2 KV and/or ±5 KV, the electric potential of the developer image transferred to the ITB will be lowered below a proper level needed for transfer or the second bias voltage will not form an electric field needed for transferring the developer image to the image receiving medium, whereby the transfer efficiency will be considerably deteriorated.
Therefore, what is needed to obtain an excellent transfer efficiency is an ITB having electric characteristics which allow a developer image having a high electric charge to be efficiently transferred while allowing the second bias voltage to be maintained in a low level not more than ±5 KV.
In addition, an important parameter that determines transfer efficiency along with the electric property of a developer image such as an electric charge in a wet type electrophotographic printer is the carrier or toner quantity of the developer image, i.e., the density.
Nowadays, a wet type color electrophotographic printer requires a developer delivery system and a density control device, which are complicated, when a low density liquid developer of not more than 3% solids is used. Thus, such a printer becomes complicated or large-sized. Therefore, in order to avoid such a problem, there is a tendency to use a high density developer (for example, of 3 to 20% solids) instead of a low density liquid developer of not more than 3% solids.
With a wet type color electrophotographic printer forming a color image in one pass process as described above, the high density liquid developer is formed on each photoconductor as a developer image having a density, which is typically in the range of 20 to 30% solids. The developer images formed on the individual photoconductors are overlappingly transferred to the ITB in the first transfer nip section and then transferred from the ITB to an image receiving medium in the second transfer nip section.
At this time, the transfer efficiency of a developer image to the image receiving medium is affected by attraction acting between toner particles and the carrier contained in the developer image and surface tension varied depending on an ITB or an image receiving medium, with which the developer image comes into contact. That is, the transfer efficiency of a developer image is greatly affected by the density of the developer image prior to being transferred to an image receiving medium in the second transfer nip section, and the physical property of an ITB for transferring the developer image between a photoconductor and an image receiving medium.
Accordingly, it is required that a developer image be capable of being maintained in a density for allowing the best transfer efficiency to be obtained without causing spreading until the developer image is transferred to an image receiving medium after the developer image has been transferred from a photoconductor in the second transfer nip section. The developer image is also required to have a physical property forming a meniscus for allowing the best transfer efficiency to be obtained when the developer image is transferred from the ITB to the image receiving medium.
Like this, in order to obtain an excellent image which does not cause a transfer defect in a wet type color electrophotographic printer using a high density liquid developer having a high electric charge, the necessity is increased for an ITB, which has electric and physical properties appropriate for transferring a high density developer image having a high electric charge.